1. Field of the Invention
This invention relates to a keratometer used in ophthalmic hospitals for measuring the shape of the cornea of an eye to be examined,
2. Related Background Art
There is known a keratometer as disclosed in Japanese Patent Publication No 1-19896 wherein three point sources of light on the circumference of the same circle including two points in a diametrical direction are projected onto the cornea of an eye to be examined, the positions of the images reflected by the cornea are detected and the center of an ellipse is found from the intermediate position of the reflected images of the two points in the diametrical direction and further an ellipse linking the three reflected images is specified to thereby measure the shape of the cornea.
However, in the above-described example of the prior art, in the process of calculating the shape of the ellipse, the information of the positions of only two of the three reflected images is used to calculate the center of the ellipse linking the three reflected images and thus, although the information of the positions of the three reflected images is measured, all the information is not utilized and the accuracy of the measured value of the shape of the cornea is reduced.
Also, three point sources of light are provided on a plane perpendicular to the optic axis of the eye to be examined and on a half of the circumference of the circle centered at the optic axis and therefore, when these point sources of light are projected onto the cornea of the eye to be examined, the projected positions incline toward one side of the cornea and the point sources of light are not at all projected onto the other side of the cornea and thus, the measured value of the shape of the cornea does not at all contain the information of that half of the cornea. Since generally the cornea is asymmetrical, this measured value of the shape of the cornea which does not at all contain the information of that half of the cornea is low in reliability. To obtain the information of the remaining half of the cornea to thereby enhance the accuracy of measurement, a total of four or more point sources of light is necessary if further point sources of light are provided. Also, the point sources of light must be provided on a half of the circumference of a circle and therefore, their design positions are considerably limited and the degree of freedom in designing the apparatus is small.